The Bones of Professor Wilson

There's a fog in the meadow. Ron Wilson is a university history professor. He discovers his back yard deck is a portal from which he can see snippets of space and time from ages long ago. The images appear to float through the meadow behind his house in fog banks that suddenly form and quickly disappear. After seeing a herd of bison in one fog, he goes out into the meadow to investigate. Much to his surprise, he discovers they left hoof prints, even though they were from a long ago time.
As he kneels beside the torn-up turf to check the hoof prints another fog bank forms around him and he is terrified when he looks up and sees a dinosaur lumbering toward him.